1. Field of the Invention
The present invention relates to a spot welding control method and apparatus.
2. Description of the Prior Art
Conventionally, spot welding has been widely used to join two metal plates (i.e, work or base material). In this spot welding, current is passed through a joint portion of the two metal plates, to join the joint portion by forming a nugget partially melted due to heat generated by resistance. In the mass production factory of automotive vehicles, in particular, a great number of welded portions are spot-welded automatically by use of a number of welder robots provided with spot welding electrodes.
FIG. 5 is a schematic block diagram showing a prior art control apparatus used for a spot welder robot. The spot welding conditions are set to a welding controller 1, and the spot welding electrodes are moved and further pressured by use of a welding pressure actuator 2. Further, when the pressuring force of the spot welding electrodes is controlled by the welding controller 1, the pneumatic pressure of an air cylinder 6 is controlled. Further, the welding pressure actuator 2 includes a programmable logic controller (referred to as PLC, hereinafter) 4 for executing the welding sequence control, a valve unit 5 electrically controlled by the PLC 4, and the air cylinder 6 controlled by the valve unit 5 on the basis of pneumatic pressure to move and pressure the spot welding electrodes. Further, the welding controller 1 includes a welding pressure controller 7 for controlling the welding pressure, and a welding current controller for controlling the welding current flowing through the spot welding electrodes.
A robot controller (electric control unit) 3 for controlling a spot welder robot includes a robot CPU 10 for controlling the robot operation in accordance with a previously determined program, in which a memory for storing welding condition data 12 can be accessed through a system bus 11. As the welding condition data 12, various welding conditions such as position data, welding time, welding pressure, welding current, etc. all necessary for spot welding are stored. Therefore, after having accessed the welding condition data 12, the robot CPU 10 moves the spot welding electrodes to a welding position of a work to be spot-welded, and sets welding pressure to the welding pressure controller 7 and welding current to the welding current controller 8, respectively both included in the welding controller 1 via an interface (referred to as I/F, hereinafter) 13 connected to the system bus 11. Here, another I/F 14 is connected to the system bus 11, so that it is possible to input or output data in and from the welding pressure actuator 2. Further, as the serial transmission signal lines 15 and 16 conforming to the RS422 standard are provided between the I/F 13 and the welding controller 1 and between the I/F 14 and the welding pressure actuator 2, respectively.
When a welding pressure is set as the welding condition data 12, the welding pressure controller 7 forms data for generating the welding pressure by the welding pressure actuator 2, and controls the pneumatic pressure of the air cylinder 6. In order to execute a highly reliable spot welding operation without defects, it is necessary to control the welding pressure according to the time stage of the spot welding and the welding current synchronized with the welding pressure systematically under optimum conditions.
The prior art technique related to the synchronous and systematic control between the welding pressure and the welding current for the spot welding is disclosed in Japanese Patent Laid-Open No. 6-226455 or No. 7-132382, for instance. In the case of the Japanese Patent Laid-Open Patent Application No. 6-226455, there is disclosed such a method that when an aluminum-lithium (Al-Li) alloy is spot-welded on the basis of resistance, a large welding current and a low welding pressure are set at the first half period of spot welding but a small welding current and a high welding pressure are set at the second half period of the same spot welding. Further, in the case of the Japanese Published Unexamined Patent Application No. 7-132382, there is disclosed such a method that a stable spot welding can be enabled, in spite of a small welding current, by switching high and low welding pressure alternately, while keeping the welding current constant or slightly increasing the welding current. In these prior art welding techniques, the welding pressure and the welding current are both changed at a plurality of the welding stages in order to form an excellent nugget at the joint portion; that is, in order that the welding can be executed under such conditions that a welding trouble such as so called "expulsion and surface flash" will not be generated.
In the prior art spot welding system as shown in FIG. 5, however, since the welding controller 1 for controlling the welding pressure and the welding current, and the welding pressure actuator 2 are both connected to each other through the system bus 11 and the two I/F 13 and 14, separately, the individual system elements are disposed separately. Therefore, there exists such a problem in that the data transmission paths are long between the respective system elements and thereby the timings required for transmission procedure are complicated, with the result that it is difficult to synchronize and execute a systematic control of both the welding current and the welding pressure during a single short time spot welding process.
Further, in the case of the spot-welded work, the joint portions of the work are not necessarily brought into tight contact with each other, but brought into tight contact with each other only after having been pressured by the spot welding electrodes. Further, when the nugget is formed by heating the resistance portion of the work by the welding current, the joint portions are thermally expanded or the melted portion and the heat generating portion are softened and thereby compressed by the welding pressure. As a result, in order to control the welding pressure of the spot welding electrodes, it is necessary to control the position of the spot welding electrodes according to change of the work status. In the case where the welding pressure is generated by the air cylinder 6 as shown in FIG. 5, when the spot welding electrodes are moved at a relatively slow speed, the welding pressure can be maintained constant. However, when the spot welding electrodes are moved at a relatively high speed, it has been difficult to control the spot-welded positions at high speed. In the case of the Japanese Published Unexamined Patent Application No. 6-226455, although the welding pressure and the welding current are both changed at two different stages, the construction of the used spot welder, in particular the construction of the pressuring mechanism of the spot welding electrodes is not shown. On the other hand, in the case of the Japanese Published Unexamined Patent Application No. 7-132382, although the welding pressure of the spot welding electrodes can be controlled by use of a welding pressure actuator including a rotation-to-straight convertor for converting the rotational motion of a motor into a straight line motion is disclosed, it is not clear the method of detecting and changing the welding pressure.